C++ Pointer Introduction

Introduction

One way to access an object in memory is via pointers.

Each object in memory has its type and an address.

This allows us to access the object through a pointer.

The pointers are types that can hold the address of a particular object.

In the following code, we will declare an unutilized pointer that can point to an int object: 

int main() 
{ 
    int* p; 
}

We say that p is of type int*.

To declare a pointer that points to a char we declare a pointer of type char*: 

int main() 
{ 
    char* p; 
}

In our first example, we declared a pointer of type int*.

To make it point to an existing int object in memory, we use the address-of operator &.

We say that p points to x. 

int main() 
{ 
    int x = 123; 
    int* p = &x; 
}

In our second example we declared a pointer of type char* and similarly, we have: 

int main() 
{ 
    char c = 'a'; 
    char* p = &c; 
}

To initialize a pointer that does not point to any object we can use the nullptr literal: 

int main() 
{ 
    char* p = nullptr; 
}

It is said that p is now a null pointer.

Pointers are variables, just like any other type of object.

Their value is the address of a memory location where the object is stored.

To access a value stored in an object pointed to by a pointer, we need to dereference a pointer.

Dereferencing is done by prepending a pointer (variable) name with a dereferencing operator *: 

int main() 
{ 
    char c = 'a'; 
    char* p = &c; 
    char d = *p; 
}

To print out the value of the dereferenced pointer, we can use: 

#include <iostream> 

int main() //from w w w  .j  a v  a 2 s. co m
{ 
    char c = 'a'; 
    char* p = &c; 
    std::cout << "The value of the dereferenced pointer is: " << *p; 
}

Now, the value of the dereferenced pointer *p is simply 'a'.

Similarly, for an integer pointer we would have: 

#include <iostream> 

int main() //from w  w w . j  av  a2  s  .co m
{ 
    int x = 123; 
    int* p = &x; 
    std::cout << "The value of the dereferenced pointer is: " << *p; 
}

And the value of the dereferenced pointer, in this case, would be 123.

We can change the value of the pointed-to object through a dereferenced pointer: 

#include <iostream> 

int main() /*w w w  .  j  a v  a2  s  .com*/
{ 
    int x = 123; 
    int* p = &x; 
    *p = 456; // change the value of pointed-to object 
    std::cout << "The value of x is: " << x; 
}

We will talk about pointers, and especially about smart pointers when we cover the concepts such as dynamic memory allocation and lifetime of an object.




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